Membrane-introduction mass spectrometry (MIMS) devices have been used to measure dissolved elements in natural and manmade fluid environments. The MIMS approach was first described by Hoch, G. and Kok B, “A mass spectrometric inlet system for sampling gases dissolved in liquid phase”, 1963, Archives of Biochemistry and Biophysics, 101:160, and numerous improvements to the method have since been described and published. A recent variation on the MIMS device is described, for example, in U.S. Pat. No. 6,727,498 to Fries et al., showing a portable mass spectrometer for underwater use that includes a watertight case having an inlet and means for transforming an analyte gas molecule from a solution phase into a gas phase positioned within the case. To date, however, no MIMS device has been described that can successfully operate to high pressures (>400 bars) and great water depths (>4,000 m).